1. Field of the invention
The present invention generally relates to devices utilizing a surface acoustic wave (SAW), and more particularly, to improvements in the temperature stability and shape factor of the SAW devices.
2. Description of the Related Art
The SAW device is widely applied to the band-pass filter and duplexer of the cellular phone. In recent years, the filter using the SAW device is required to have higher performance as the cellular phone has higher performance. One of the requirements for the SAW device is to improve the temperature stability because temperature change moves the pass-band frequency range of the filter. As is known, lithium tantalite (LiTaO3, hereinafter simply referred to as LT) is a piezoelectric material having a large electromechanical coupling coefficient, which is advantageous for realizing broad filter characteristics. However, LT has a disadvantage in that it is inferior to quartz crystal in terms of temperature stability. The piezoelectric material has a general tendency of incompatible characteristics such that materials having large electromechanical coupling coefficients such as LT and lithium niobate (LiNbO3, hereinafter simply referred to as LN) have comparatively poor temperature stability, while materials having good temperature stability such as quartz crystal have comparatively small electromechanical coupling coefficients. Thus, a piezoelectric material having both a large electromechanical coupling coefficient and a good temperature stability has been sought for years.
There are various proposals to realize materials having a large electromechanical coupling coefficient and a good temperature ability. For example, Yamanouchi et al. propose a substrate having an LN or LT base and a SiO2 film, which is grown thereon and has a temperature coefficient opposite to that of LN or LT (see IEEE Trans. On Sonics and Ultrasonics., vol. SU-31, pp. 51-57, 1984). Nakamura et al. achieve improvements in temperature stability by forming a polarization inverted region in the LT substrate surface that has a depth less than the involved wavelength and utilizing the electrostatic short-circuit effect thereof (see Japanese Patent No. 2516817). Onishi et al. propose to improve the temperature stability by directly joining a thin piezoelectric substrate and another substrate made of a thick, low-expansion material and thus suppressing contraction and expansion due to change of the temperature of the piezoelectric substrate (see Japanese Laid-Open Patent Application No. 11-55070, Proc. Of IEEE Ultrasonics Symposium, pp. 335-338, 1998). Yamanouchi et al. propose to use adhesive for joining the two substrates (see, Yamanouchi et al. The 20th Symposium, November, 1999).
As described above, there are various proposals to realize the SAW device having a large electromechanical coupling coefficient and good temperature stability. However, the proposals have problems to be solved. The proposal by Yamanouchi et al. has difficulty in controlling the thickness of the SiO2 film and thus obtaining a desired constant film thickness and large SAW propagation loss and. The proposal by Onishi et al. requires the piezoelectric substrate and the low-expansion substrate to have mirror surfaces for joining and causes the bulk wave to be reflected at the joining interface, this degrading the filter characteristics. Yamanouchi et al. also propose to make the back surface of the piezoelectric substrate coarse and join the low-expansion material to the coarse back surface by adhesive (Japanese Laid-Open Patent Application No. 2001-53579). However, this proposal decreases the adhesive force at the joint interface and reduces the degree of improvement in temperature stability.